The application of siloxane compositions to provide coatings on various substrates for example to improve the release of adhesive and other materials therefrom has been practised for many years. Although the conventional siloxane release coating compositions normally perform satisfactorily in use there is a continuing demand for improved compositions. In particular a need has been expressed for compositions which cure faster and which require the application of less energy for the curing step. Also if such compositions could be applied in the absence of a solvent or other diluent this would represent an additional advantage.
British Pat. No. 1 553 586 discloses radiation-curable polydiorganosiloxane compositions consisting essentially of a mixture of (A) a triorganosiloxy-endblocked polydiorganosiloxane fluid wherein from 1 to 5 percent of all radicals are mercaptoalkyl having the formula EQU --(CH.sub.2).sub.n SH
wherein n has a value of from 1 to 4 inclusive; and (B) a methylvinylpolysiloxane compatible with (A) and having an average of at least 3 silicon-bonded vinyl groups per molecule. The compositions may also contain a photosensitising compound. Compositions of this type can be applied in the solventless condition to a substrate and cured rapidly to provide non-adhesive coatings. However, it has been found that the coatings often exhibit a slight but disagreeable odour after curing especially if the coated substrate, e.g. paper, is stored in the rolled up state.
In British Pat. No. 1 569 681 there are described compositions which are curable by ultra-violet radiation and which comprise (a) an organopolysiloxane having in the molecule at least one mercaptoalkyl-containing organosiloxane unit, (b) an organopolysiloxane having in the molecule at least one allyl-containing organosiloxane unit and (c) a photosensitiser. The patent also discloses the use of the said compositions for providing release coatings on paper. Although the odour associated with the compositions of Pat. No. 1 569 681 is not as marked as that referred to hereinabove the allyl-siloxane component is difficult and relatively expensive to prepare.